1. Field of the Invention
This invention is an improved variable-frequency drive for single-phase AC motors.
2. Prior Art
Prior art involves various approaches for variable-frequency drives for single-phase induction motors. The first approach uses an inverter to supply a single-phase, sinusoidal output of variable voltage and frequency. The voltage is normally reduced in proportion to the frequency.
While this approach can give some variation in motor speed, it has several problems. One problem is that the impedance of the run capacitor increases as the frequency is reduced. This high impedance reduces current in the start winding, which means that the torque of the motor is not sufficient to start the motor at low frequency. In addition, the prior art uses a filter to remove the high-frequency components from the power supply, which adds to the cost and complexity of this approach.
The second approach uses a two-phase inverter. This setup removes the run capacitor from the circuit and supplies the start winding with a voltage that is 90.degree. out of phase with voltage for the main winding. Since the run capacitor is normally sized so that the voltage across the start winding is higher than that for the main winding, the inverter must also supply a higher voltage to the start winding.
There are two problems with this approach. The first problem is that the inverter must create a voltage that is higher than the utility line voltage in order to supply the start winding, which adds cost and complexity to the electronics. The second problem is that the required voltage for the start winding varies from motor to motor so that field retrofit of a motor is difficult.